Artificial joint, in particular endoprosthesis for replacing natural joints

ABSTRACT

The invention concerns an artificial joint, in particular an endoprosthesis for replacing natural joints, comprising at least two artificial joint parts with curved articulation faces, a curved contact line being formed on each of the articulation faces. The curved contact line (L 1 ) of one of the articulation faces is part of an elliptical section contour of a first cylinder ( 1 ) or cone having the cylinder radius (R 1 ) or the cone angle (α1). The other contact line (L 2 ) takes the form of a counter track of a second cylinder ( 2 ) or second cone having the cylinder radius (R 2 ) or the cone angle (α2) and rolling and/or sliding on the first cylinder ( 1 ) or first cone. The articulation faces comprise control faces (F 1 , F 2 ) formed from a plurality of straight contact lines (B). On one side, these control faces (F 1 , F 2 ) adjoin the contact lines (L 1 , L 2 ) opposite one another, and each of the contact lines is the connection line between an instantaneous contact point (K) of the contact lines (L 1 , L 2 ) and an instantaneous common point (Q) or the instantaneous pole of the respective movement systems in a reference plane (X, Y) or a reference sphere in the moved/unmoved system.

The invention presented relates to an artificial joint, in particular anendoprosthesis for replacing natural joints, comprising at least twoartificial joint parts, each with curved articulation faces on which thejoint parts articulate with respect to one another.

Such an artificial joint is known from the German patent application P42 02 717.9. In it, the joint faces have different circular sectioncontours in planes which are perpendicular to each other, namely onelongitudinal plane and a transverse plane, where the ratios of curvaturefor the articulation faces in each of the planes are convex—convex,convex-concave or concave—concave, and the geometry of the joint'sarticulation faces with respect to one another is determined in each ofthe two planes by a joint chain with two joint axes, a so-called dimericjoint chain, which runs through the centers of curvature of thearticulation faces of their respective section contours. Since thearticulation faces of this artificial joint are formed convex-concave,concave—concave or convex—convex, essentially point-like loadtransmission regions come into being so that increased surface pressurescan occur on the articulation faces which lead to wearing away ofmaterial. This can shorten the life span of these artificial joints. Inorder to improve the load transmission between the articulation faces ofthe parts of the joint, it is suggested in the known joint to arrange apressure distribution body between each of the individual parts of thejoint with which a better and more even distribution of pressure isachieved. However, this pressure distribution body increases the numberof required joint parts for the artificial joint.

The object of the invention presented is to create an artificial jointin which point-like load transmission regions are avoided and which itis not necessary to fit with pressure distribution bodies and which atthe same time conforms optimally to the conditions of the human bodywhen used as an endoprosthesis, particularly substituting a naturalhuman joint.

According to the invention, this is achieved with an artificial joint ofthe kind described in the introduction, where on each of thearticulation faces a curved contact line is formed, and the curvedcontact line of one of the articulation faces is part of an ellipticalsection contour of a first cylinder or a cone having the cylinder radiusR1 or respectively the cone angle α1, and the other contact line takesthe form of a counter track of a second cylinder or second cone havingthe cylinder radius R2 or respectively the cone angle α2 and rollingand/or sliding on the first cylinder or first cone; also thearticulation faces comprise control faces formed from a plurality ofstraight contact lines, and, on one side, these control faces adjoin thecontact lines opposite one another, and each of the contact lines is theconnection line between an instantaneous contact point of the contactlines and an instantaneous reference point of the respective movementsystems in a reference plane or on a reference sphere in themoved/unmoved system. In accordance with the invention, a fixed ormoving point of the moved or unmoved system will preferably be chosen asreference point, where it is assumed that one of the cylinders or conesis stationary and the other cylinder or cone rolls and/or slides on thisstationary cylinder/cone.

The restriction to an unavoidable motion and the choice of theinstantaneous axis of rotation as reference point cause, since theholoids roll on each other without sliding, this characteristic to betransmitted to corresponding sections of the control faces. If, insteadof the instantaneous center of revolution, another instantaneous commonpoint is used from the reference plane or reference sphere in the movedor unmoved system, the ratio of rolling to gliding can thereby bevaried.

According to the invention, it is further provided that the first andthe second cylinder or the first and the second cone are arranged withrespect to each other so that they form a straight dimeric joint chainor an overlaid dimeric joint chain. For cylinders in a straight dimericchain, the relationship R=R2+R1 applies or, in an overlaid dimericchain, the relationship R=R2−R1, where R is the radius of the joint'saxial path and R1 the radius of the first cylinder and R2 the radius ofthe second cylinder. In the case of the cone, analogous to thecylinders, α=α2+α1 and α=α2−α1 apply, where α is each of the anglesbetween the axes of the tangential cone pairs.

Since, according to the invention, the cylinders or cones roll or slideon or in each other and the distance between the cylinder axes or theangle between the cone axes remains constant, a flat or sphericaldimeric chain results. Thus, the motion, in principle having threeparameters and possible as flat or spherical, is limited to two degreesof freedom. The corresponding radii of the cylinder or angles of thecone are preferably adapted to the anatomical conditions of the humanknee joint, however can also be changed according to the materials usedand their characteristics.

The artificial joint according to the invention distinguishes itself inthat a linear load transmission region is formed in every contactposition of the articulation faces.

Furthermore, it can be advantageous according to the invention if nextto the region of direct load transmission a region without any touchingcontact is formed in the region of the control faces, such that inaction the surrounding tissue is only minimally injured. Therefore,according to the invention it is advantageous if, on the side of thecontact lines opposite the control faces, the contact lines are extendedarched in bowed lines so as to form toric surfaces. The curved lines aredefined in that at the instantaneous contact point of the contact linesin the moved or unmoved system a plane is formed which stretches fromthe respective contact line of the control faces and the commonperpendiculars of the contact lines to a tangent of one of the contactlines at the respective contact point. The toric surfaces are formedsuch that the outer part of the toric surfaces' articulation faces don'ttouch each other. These outer parts of the toric surfaces form thecurved articulation faces' region of indirect load transmission.

According to the invention, the curved lines are attached to theirrespective contact lines of the control face without any kink, and theirnormal lines coincide at the contact point. By advantageously formingthe curved lines in circularly arcuate sections—if differently sizedradii are chosen in the outer sections—a free space can always bemaintained between the articulating toric surfaces during the moving ofthe articulation faces.

Preferably, the joint according to the invention is used in a four-jointas an endoprosthesis for the human knee such that the medial jointcompartment forms the overlaid dimeric chain, and the lateral jointcompartment the straight dimeric chain, whereby a four-joint isinevitably formed.

Using the exemplary embodiments shown in the enclosed drawings, theinvention will be explained in detail.

BRIEF DESCRIPTON OF THE DRAWINGS

FIG. 1 is a schematic diagram of a joint comprised of two cylindersarranged in a flat, straight dimeric chain;

FIG. 1a is a schematic diagram of a joint comprised of two cones rollingon each other in a dimeric joint chain;

FIG. 2 is a schematic diagram of a joint comprised of two cylindersarranged in an overlaid dimeric joint chain;

FIG. 2a is a schematic diagram of a joint comprised of two cones rollingone inside the other in an overlaid dimeric joint chain;

FIG. 3 is a diagram showing the generation of control faces in the jointarrangement of FIG. 2;

FIG. 4 shows the control face F1 produced by the sum of the contactlines B in a resting system;

FIG. 5 shows the control face F2 produced by the sum of the contactlines B in a moving system;

FIG. 6 is a diagram illustrating the construction required forproduction of articulating partial faces according to the invention;

FIG. 7 is an illustration of an articulating toric surface generated bya curved extension of the contact line B in the moved system;

FIG. 8 is an illustration of an articulating toric surface generated bya curved extension of the contact line B in the unmoved system;

FIG. 9 shows the articulation surfaces of a joint head of an artificialjoint incorporating the control surface F1 of the unremoved system withthe connected toric surface 10 according to FIG. 8;

FIG. 10 shows the femoral articulation surfaces of a joint head of anartificial joint incorporating the two control sufaces F1 of the unmovedsystem with connected toric sufaces 10 according to FIG. 8 on oppositesides of a mid-plane X—X; and

FIG. 11 shows the tibial articulation surfaces of a joint head of anartificial joint incorporating the two control surfaces F2 of the movedsystem with connected toric surfaces 9 according to FIG. 7 on oppositesides of a mid-plane X—X.

According to the present invention, a particular flat or also sphericalcontrolled movement should be achieved. This is achieved in that a flator spherical linkage is provided. In this linkage, the rotational axesare replaced in a first step by cylinders, arranged around these axes,which touch each other or by circular cones, where these cylinders orcones roll and/or slide on each other. The corresponding radii of thecylinders or the cone angles of the cones are adapted to the anatomicalconditions of the natural joint to be replaced, in particular the humanknee joint.

In FIG. 1 two cylinders 1, 2 are shown which roll and slide on eachother and have the midpoints M1 and M2 and the radii R1 and R2 and therotational axes D1 and D2. The pair of cylinders shown is arranged as aflat, straight dirneric chain, so that cylinder 2 rolls or slides oncylinder 1. R=R2+R1 applies, where R is the radius of the joint's axialpath, also the length of the dimeric joint chain. FIG. la shows arepresentation of two cones 1, 2 rolling on each other and having theirrespective cone angles α1, α2, where α=α2+α1 applies.

In FIG. 2 a cylinder arrangement is shown of cylinders 1 and 2,midpoints M1 and M2, radii R1 and R2 and cylinder axes D1 and D2, wherethese cylinders are arranged in the form of an overlaid dimeric chain.Here, R=R2—R1 applies, where R is again the radius of the joint's axialpath and therefore the length of the dimeric chain. FIG. 2a shows arepresentation of two cones 1, 2 rolling inside each other and havingtheir respective cone angles α1, α2, where α=α2−α1 is.

As shown in FIG. 3, in this arrangement, for example, cylinder 1 ischosen as the stationary piece, where, by cutting through cylinder 1 atan angle, a contact line L1 is chosen on cylinder 1 which thus has anelliptical form. Cylinder 2 is chosen as the moving piece and rollsand/or slides now on cylinder 1, where on the cylinder surface acounter-curve forms as contact line L2 as a function of the rolling andsliding motion of cylinder 2. This contact line 2 has a arcuated form,seen section-wise. Therefore, both contact lines L1 and L2 have at anygiven instant a common contact point K. According to the invention,cylinder 1 can, for instance, be assigned as the femural joint piece ofthe inventive knee in the construction of an artificial joint, andcylinder 2 as the tibial joint part.

In FIG. 3 it is shown how now, in the joint arrangement according toFIG. 2, based on contact lines L1 and L2, which are formed on cylinders1 and 2, contact lines B, which require linear contact, are produced forgenerating control faces according to the invention. Here, a basis pointQ is advantageously fixed in the moved system, being randomly chosen ina reference plane (sagittal plane) that is shifted to the interior ofthe joint such that at the instantaneous contact point K of the contactlines L1 and L2 the connection line B advantageously forms an angle β(35°<β<70°) with respect to the z axis. This reference plane liesparallel to the functional plane, here represented by the coordinateplane x and y. The sum of the contact lines B in the resting systemproduces a control face F1 having lines running between contact line L1and path 5 of the basis point Q (see FIG. 4). In the moved system, acontrol face F2 arises between contact line L2 and basis point Q,stationary in this system (see FIG. 5). Therefore, in every state ofmotion, both control faces F1 and F2 each move linearly in accordancewith their design. For each of the joint surfaces a part of the controlfaces F1 and F2 is chosen which adjoins L1 or L2 and extends a maximumof 3 cm, measured from the contact line to the middle. These parts ofthe control faces F1 and F2 formed by the contact lines B belong to theregion of direct load transmission.

Furthermore, it is provided according to the invention that articulatingpartial faces with indirect load transmission are produced on the sideof the contact lines L1, L2 opposite the control faces F1, F2. Thedesign required for that and in accordance with the invention isexplained using FIG. 6. At the instantaneous contact point K of thecontact lines L1, L2, an instantaneous plane is set up as an aide in themoved and the unmoved system and stretches from the contact line B ofthe control faces F1, F2 and a common perpendicular 7 of the contactline B and a tangent t of the contact line L1 and/or L2. In this plane,arcuate curves, curved lines S1, S2, which advantageously remain thesame throughout their progression, are attached without any kink to thecontact line B of the control faces F1, F2. In the moved system, as isshown in FIG. 7, and in the unmoved system, as is shown in FIG. 8, toricsurfaces 9, 10 are thereby produced which are attached to theirrespective control faces F1, F2 without any kink and whose normal linescoincide at the contact point. The arcuate curves S are made such thatduring motion free space always remains between the articulating toricsurfaces 9, 10 which they form. This can be achieved with the circularbowed lines S1, S2 by choosing different sized radii. Furthermore, itcan be advantageous to provide S1 and S2 with a common radius over acertain distance (up to maximum 2 cm) and only then, without any kink,to introduce differently sized radii. The region of direct loadtransmission is thereby extended into the arcuate region. Since thecontrol faces F1 and F2 and the toric articulation faces 10, 9 comeabout due to the motion of an elliptical section contour comprised oflines and curves, these faces can be produced using a CNC grindingmachine. Basis point Q can also be defined in the unmoved system. It canfurther be advantageous to choose Q depending on the motion andparticularly to use for this the instantaneous center of revolution P(FIG. 6) which lies in a random intermediate plane. The control face F1lies, then, between contact line L1 and the resting pole path 4, and thecontrol face F2 lies between contact line L2 and the moving pole path 6.Resting pole path 4 and moving pole path 6 arise as intersection linesof this randomly chosen plane with the sum of the instantaneousrotational axes of the motion. If the instantaneous center of revolutionis chosen to produce the contact lines B, sliding is minimized on theformed control faces F1 and F2 since the holoid surfaces roll on eachother. If, instead of the instantaneous center of revolution, anotherinstantaneous common point in a reference plane in the moved/unmovedsystem is used, the ratio of rolling and sliding can be varied.

FIG. 9 shows the transfer of the unmoved system's configuration from thecontrol face F1 with attached toric surface 10 according to FIG. 8 to anartificial joint part 12 which can form the articulation face of ajoint's head. It is to be noted here that the dimensions of the faces F1and 10 are adapted to the anatomical conditions.

A joint according to the invention is formed advantageously as anendoprosthesis for replacing the human knee joint from by coupling twojoint arrangements according to the invention as shown in FIGS. 1 and 2.Here, the control faces of each pair of joint bodies are arranged withrespect to a mid-plane X—X such that they lie on the side facing themid-plane X—X. The tibial joint body and the femural joint body here areeach fixed immovably with regard to each other. Thus, a compulsorycharacteristic arises which is determined by the resultant four-joint.In the side view the instantaneous center of revolution results as anintersecting point of the lateral and medial chain (or theirextensions). In all, the resting pole path is created in the stationaryplane and the moving pole path is created in the moved plane. For a kneejoint of the right knee seen from behind, FIGS. 10 and 11 show thefemural articulation faces and tibial articulation faces of the lateraljoint compartment 11 and the medial joint compartment 12 which areproduced according to the design process in accordance with theinvention. In the regions of the indirect load transmission, that is thetoric articulation faces 9, 10, the radii are chosen such that tibialsized radii are provided for femurs. In the lateral joint compartment11, an arrangement according to FIG. 2 is formed, namely a straightdimeric joint chain, and in the medial joint compartment 12 an overlaiddimeric joint chain is provided according to FIG. 1.

What is claimed is:
 1. An artificial joint for use as an endoprosthesisfor replacing a natural joint, said artificial joint comprising at leasttwo artificial joint members each having curved articulation faces;wherein said joint is a flat or spherical four joint comprising firstand second sides on opposite sides of a mid-plane; wherein on each sideof the joint a curved contact boundary is formed on each of thearticulation faces; the curved contact boundary on one of thearticulation faces being part of an elliptical section contour of afirst cylinder or cone representing a fixed joint member, and the curvedcontact boundary on the other of said articulation faces having the formof a counter track of a second cylinder or cone representing a movedjoint member which rolls or slides on the first cylinder or cone; eachside of said joint having a fixed or moving holoid lying in a referenceplane or sphere, said reference plane or sphere being selected such thatit lies between the condyles to be constructed; wherein a straightcontact line along which the fixed and moved members touch at any giveninstant is drawn from a respective contact point lying on the curvedcontact boundaries either to a point which is fixed relative to thefixed or moved member and which lies in a randomly selected sagittalplane shifted toward the interior of the joint, or to the instantaneouscenter of revolution on the fixed or moving holoid; and wherein the sumof the straight contact lines forms a first contact face on the fixedmember and a second contact face on the moved member, said first andsecond contact faces each representing a joint face of one of the twosides of the four-joint.
 2. An artificial joint according to claim 1,wherein the curved contact boundary on one of the articulation faces ispart of an elliptical section contour of a first cylinder having aradius R1 and the curved contact boundary on the other of saidarticulation faces is a counter track of a second cylinder having aradius R2, and wherein the first and second cylinders are arranged withrespect to each other such that they form a stretched dimeric link chainhaving an axial path radius R which satisfies the relationship R=R2+R1or an overlapped dimeric link chain having an axial path radius R whichsatisfies the relationship R=R2−R1.
 3. An artificial joint according toclaim 1, wherein the curved contact boundary on one of the articulationfaces is part of an elliptical section contour of a first cone having acone angle α1, the curved contact boundary on the other of saidarticulation faces is a counter track of a second cone having a coneangle α2, and wherein the first and second cones are arranged withrespect to each other such that they form a stretched dimeric link chainhaving an angle α which satisfies the relationship α=α2+α1 or anoverlapped dimeric link chain having an angle α which satisfies therelationship α=α2−α1.
 4. An artificial joint according to claims 1,wherein the straight contact lines have curved extensions on the side ofthe curved contact boundaries opposite the contact faces such that toricsurfaces are formed; said curved extensions lying in a plane defined bythe respective straight contact line and a line perpendicular to atangent (t) of the curved contact boundary at the respective contactpoint, and wherein the toric surfaces are formed such that outerportions of the articulation faces of the toric surfaces have no contactwith each other.
 5. An artificial joint according to claim 1, whereinsaid joint comprises an overlapped dimeric link chain forming a medialjoint compartment of an artificial joint for a human knee, and astretched dimeric link chain forming a lateral joint compartment of saidartificial joint, and wherein the respective joint compartments comprisefemur-side joint parts and tibia-side joint parts which are rigidlyconnected with respect to each other.